In the installation of piping systems, it is often necessary to butt weld together long lengths of pipe. This limits access to the internal "root" side of the weld joint. As used herein, the term "pipe" also includes tubular metal parts where there is limited access to the internal root side.
Highly alloyed pipe, for example, nickel-base superalloys and corrosion resistant alloys, is especially subject to welding defects at the "root" side of the weld even when gas back-purging is used. Typical pipe may also include highly alloyed iron-base alloys and cobalt-base alloys.
Efforts to solve the problem are found in the prior art. For example, U.S. Pat. No. 3,736,400 (Spiegel, et al) discloses the use of water soluble paper to construct a protective dam. U.S. Pat. No. 3,292,254 (Sloan) discloses a bag-like collapsible container as a means to control the atmosphere. American Welding Society 1987 publication ANSI/AWS D10.11-87 entitled "Recommended Practices for Root Pass Welding of Pipe Without Backing" also discusses the problem in a very comprehensive state-of-the-art manner. In the March 1986 issue of Welding Design & Fabrication, page 59, an article entitled "Power-Plant Pipe Must be Leak Free" also describes the process.
An article by P. Aastrup and P. Arp entitled Welding in Stainless Steels with Underside Gas Shielding was published in SVEJSNING Vol. II, Part 1, February 1984 (in Swedish). The article describes several degrees of oxidation of the weldment during welding and discloses methods to control (reduce) oxidation rates by gas back-purging.
In every case, the prior art mentioned herein, and all other references known to date, consistently teach that a very low flow rate of gas back-purging should be maintained in the root section during welding. One reference suggested "barely detectable at the gas exit port." For example, 8 to 12 cubic feet per hour (CFH) is suggested in the ASM Handbook Vol. 6, 9th Edition, page 200.
In this welding process, open-butt welding, using the gas tungsten arc welding (GTAW) process, is normally employed in making root-pass welds. When these one-sided welding techniques are used, it is necessary to protect the root side of the weld joint from oxidation during welding with an inert shielding gas (usually welding grade argon). The shielding technique, which protects the inside of the pipe, is called back-purging.
Lack of proper shielding produces a condition often referred to as "sugar" of the root pass. Such a condition is characterized by a black, crusty appearance of the weld metal on the inside (root) of the pipe. When this occurs, the molten weld metal will not flow out and "wet" the base material edges. This condition makes welding difficult and the passing of a radiographic examination nearly impossible. Common welding defects, which form as a result of this condition, include lack of fusion, root-pass cracking and root-pass suck-back.
It has been reported that, when using this one-sided technique, nickel-base alloys, such as produced under the trademark HASTELLOY, are more difficult to weld than stainless or carbon steels.
Two common methods are used to back-purge a pipe root-pass weld zone. Purging the entire volume of a long pipe run is often suggested. Tables have been published concerning purge times for various pipe diameters and lengths of pipe run. Local purging of the immediate volume around the weld zone is an alternative method of back-purging and is generally considered the preferred method. Various methods of forming the closure dams are reported in the literature. They include soluble dams, inflatable bladder dams, collapsible disk dams and thermally disposable dams.
Back-purging is accomplished in two steps. First, the dams must be installed and the weld-zone volume purged to acceptable levels. Second, purging must continue and accompany the actual welding operation.
Even with the best practice of the present state-of-the-art process, the results are erratic and often major failures occur. The cause of the erratic results has not been determined.